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Abstract
Ubiquitous declines in biochemical reaction rates above optimal temperatures (Topt) are normally attributed to enzyme state changes, but such mechanisms appear inadequate to explain pervasive Topt well below enzyme deactivation temperatures (Tden). Here, a meta-analysis of 92 experimental studies shows that product formation responds twice as strongly to increased temperature than diffusion or transport. This response difference has multiple consequences for biochemical reactions, such as potential shifts in the factors limiting reactions as temperature increases and reaction-diffusion dynamics that predict potential product inhibition and limitation of the reaction by entropy production at temperatures below Tden. Maximizing entropy production by the reaction predicts Topt that depend on enzyme concentration and efficiency as well as reaction favorability, which are patterns not predicted by mechanisms of enzyme state change. However, these predictions are strongly supported by patterns in a meta-analysis of 121 enzyme kinetic studies. Consequently, reaction-diffusion thermodynamics and entropy production may constrain organism performance at higher temperatures, yielding temperature optima of life that may depend on reaction characteristics and environmental features rather than just enzyme state changes.
Highlights
Understanding the response of organisms to changes in temperature is fundamental in the life sciences
then decline above optimal temperatures (Topt) may depend on reaction characteristics as much or more than molecular state changes, as Topt often changes by 10–20 °C in response to changes in enzyme efficiency[14,15] or concentration[16,17]
An alternative mechanism of temperature dependence in biochemical reactions, in which no state changes in enzymes or their state transitions need occur, is if temperature sensitivity of product formation at reaction sites is different from that of diffusion or transport of substrates, products and heat
Summary
Reaction rates and diffusion and transport are commonly found to increase exponentially with temperature. Following Equations (1–3) and the difference in estimated ED and EZ (Fig. 1), it can be hypothesized that, as temperature increases, limits to the diffusion or transport of products (and for exothermic reactions, heat) may lead to accumulation of products and decrease reaction rate. The inclusion of diffusion or transport, and the difference in temperature sensitivity with product formation, leads to a hypothetical shift in the factor limiting the reaction rate from catalysis by enzymes (k, Equation (3)) to diffusion or transport (D, Equation (3)), to entropy production (Equation (2)) as temperature increases (Fig. 2A). Equation (8) shows that the entropy-production-limited reaction constant kS will decrease with increasing temperature because substituting the steady-state activity into Equation (2) yields kS ∝ e−ΔE/RT. Equation (9) shows that, as hypothesized, substrate concentrations and reaction rate at steady-state may become limited by diffusion or transport at higher temperatures rather than by enzyme-substrate binding and product formation,.
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